This invention relates to a frequency discriminator, which may be used either in a frequency modulator or a frequency demodulator.
In general, a frequency modulator comprises an oscillator for generating a carrier signal that is subjected to frequency modulation. Although the carrier signal should have a predetermined carrier frequency, it is difficult with a usual oscillator to achieve a sufficient frequency stability. A controllable oscillator is, therefore, used in an automatic frequency control loop together with a frequency discriminator operable in a certain frequency range to provide an automatically stabilized carrier frequency. In a simple model, the frequency stabilization is carried out by setting the zero or reference frequency of the frequency discriminator at the nominal frequency of the controllable oscillator. Those circuit constants of the frequency discriminator which determine the center frequency of the operable frequency range thereof are, however, subject to variation that results from long-term changes and, above all, changes in the ambient temperature. Due to the temperature dependency of the circuit constants, the highest possible frequency stability of the carrier frequency is about 20 PPM/.degree.C. In an improved model, the controllable oscillator output signal is once mixed with a standard output signal of a crystal oscillator into a frequency-converted signal of a lower frequency so as to enable the frequency discriminator to operate in a lower frequency range. This reduces the temperature dependency of the frequency discriminator center frequency to provide as high a frequency stability as 0.5 PPM/.degree.C. In another improved model, use is made of a crystal oscillator for generating a standard signal and a pair of frequency converters supplied with the inphase and the quadrature phase standard signals, respectively. It is, thereby possible to carry out the frequency stabilization by the use of a phase variation that appears in the frequency-converted signals of a lower frequency with a variation, if any, in the carrier frequency. Although excellent in respect of the frequency stability, the improved models are objectionable in complexity of the circuitry and bulkiness of the equipment.